An oil and gas well is created by drilling a wellbore on a desired surface site that extends from the surface to a certain depth or distance into the ground. The wellbore penetrates the underlying earth and various geologic units therein. With proper planning and placement, one or more of the geologic units penetrated by the wellbore will include commercial quantities of hydrocarbons such as oil and gas. The wellbore can extend vertically, at an angle and/or horizontally through the earth. For various reasons, including rock and drilling heterogeneities, the actual direction of a wellbore tends to deviate at least to some extent from the intended direction of the wellbore. Also, the diameter and roughness (or rugosity) of the resulting borehole typically changes as the wellbore is drilled because of similar rock and drilling heterogeneities.
As the wellbore is being drilled, a drilling fluid, also referred to as drilling mud, is continuously circulated from the surface through the wellbore and back to the surface. The drilling fluid functions to remove cuttings from the borehole, control formation pressure, and cool and lubricate the drill bit. After the wellbore is drilled to a certain or target depth, casing (typically metallic casing) is usually inserted and cemented in place in the now completed wellbore. The casing typically extends to the total depth of the wellbore. The casing isolates and seals off various geologic zones that have been penetrated by the wellbore and serves multiple other purposes. Cement material is usually injected around the casing and allowed to harden into an annular sheath around the casing. The cement sheath physically supports, positions and protects the casing in the wellbore and bonds the casing to the walls of the wellbore such that the undesirable migration of fluids between zones or formations penetrated by the wellbore is prevented.
After the wellbore is drilled to the desired depth and cased, the well is ready for the final completion and production phases. Final completion of the well includes the creation of one or more access conduits (for example, perforations) that extend through the casing and cement sheath to provide communication between the wellbore and one or more of the geologic units from which hydrocarbons are to be produced. The casing and cement sheath provide a solid support for the access conduits. Once the well is completed, the gas and/or fluids, which may include hydrocarbons and water, are produced or allowed to flow from the now completed geologic unit(s) into the wellbore and then to the surface where they are processed for future use.
Numerous important procedures are typically carried out on a well during the well drilling phase and before the well completion phase. One of these procedures involves gathering geologic and engineering data regarding the size and configuration of the borehole and the nature and characteristics of the surrounding geologic units. The collection of such data, typically referred to as well logging or formation logging, can be performed by one of several downhole methods within the uncased wellbore, including mud logging, wireline logging with a wireline cable, or using the bit assembly for measurement-while-drilling (MWD) or logging-while-drilling (LWD) techniques. Various specialized logging tools have been developed for use in connection with each method. The particular method and type of tools utilized will depend on several factors, including the borehole inclination and condition, costs and time, and the type of geologic units penetrated by the wellbore.
In one downhole logging method, a logging tool is attached to the end of a wireline cable and lowered to the desired depth in the wellbore (for example, to the bottom of the wellbore) and then pulled back to the surface at a set rate or speed (the “logging speed”). Data is collected as the tool is pulled back to the surface and transmitted through the cable to the surface. In lieu of the wireline cable, for example, another downhole tool can be used to lower the logging tool into the wellbore and pull the logging tool out of the wellbore. The data is usually collected in a spatially-corrected fashion to increase the amount of true signal over the background noise. In order to make it easier to use, the data is typically also sampled at a particular sampling rate.
Well logging tools have been around for decades. For example, when well logging first began in the early 1900's, only spontaneous or ionic potential and resistivity data was recorded. Today, there are many different types of logging tool configurations available. Examples include spontaneous potential logging tools, resistivity/conductivity logging tools, image logging tools, acoustic logging tools and density/neutron logging tools. Most of the available logging tools are limited to use in an open-hole environment, although certain types of resistivity/conductivity logging tools and density/neutron tools can be used in both an open hole and a cased hole environment. The type of data collected and the manner in which it is collected varies from tool to tool.
An example of a modern logging tool is an image logging tool. An image logging tool is used to produce “images” of the borehole wall and the surrounding geologic units penetrated by the wellbore. For example, an image logging tool can be used to identify the dip and azimuth of the geologic units around the wellbore, locate rock breakouts within the borehole, identify fractures in the surrounding geologic units and determine the composition of the surrounding geologic units. Based on the data collected, a useful well image log can be created that represents the surface of the surrounding geologic units in the wellbore.
There are many factors that can alter the quality of the data collected and recorded by an image logging tool, including the logging speed, the sampling rate, the rate of turning or spiraling of the logging tool in the hole, the borehole contact with the sensor assemblies, the proximity of the sensor assemblies to the rock surface, the borehole internal diameter, the borehole shape or rugosity, the borehole inclination, the radial arrangement of the sensor assemblies, the number and orientation of the sensor assemblies, and the sensitivity of the sensor assemblies. For example, the logging speed, sampling rate and orientation at which data is collected can be particularly important factors. Based on the dip and azimuth of the wellbore and the surrounding rock, it may be determined, for example, that the final location to which the wellbore is drilled needs to be changed and that the current wellbore needs to be re-drilled, or even that an additional wellbore needs to be drilled from a different location on the site in order to effectively and efficiently penetrate the most promising geologic unit(s).
Well logging tools, including image logging tools, can be classified in many ways, including but not limited to active vs. passive, pad vs. non-pad, statistical vs. non-statistical, and centered vs. offset or eccentric. For example, an active well logging tool emits a signal (for example, nuclear radiation, energy waves or high energy particles) into the wellbore and the surrounding geologic units in order to induce a return signal that can be received and recorded by the same tool for later processing into useful data. A passive well logging tool, on the other hand, merely receives emitted signals that contain the useful information from the geologic units penetrated by the wellbore. A passive well logging tool does not emit a signal into the wellbore or geologic units.
The types of image logging tools in use today include micro-resistivity logging tools, acoustic logging tools, and optical logging tools. All of these tools are suitable for use in an open-hole environment. A micro-resistivity image tool is an active, non-statistical image logging tool that measures the conductivity/resistivity of rock minerals, fluids, gases and other materials in a geologic unit. An acoustic image logging tool is an active, non-statistical image logging tool that uses sonic waves that reflect off rock, fluid and other material surfaces. An optical image logging tool is an active, non-statistical image logging tool that uses cameras to image the rock, fluid and other material surfaces. Micro-resistivity image logging tools are the most common and widespread image logging tool in use today. All the major logging vendors have at least one micro-resistivity imager in their portfolio.
A micro-resistivity image logging tool uses a signal transmitter to emit a measured amount of electrical current through the borehole wall into the geologic units surrounding the wellbore. Multiple signal transmitters positioned around the tool to cover the entire area surrounding the wellbore are typically used. The current emitted by each signal transmitter is altered by the conductivity/resistivity of the rock minerals, fluids, gases and other materials that are adjacent to the wellbore. The altered current is then received by a corresponding return signal sensor assembly attached to the logging tool. For example, the signal transmitters and return signal sensor assemblies can be placed in pads that are forced against the rock wall by extendable offset arms.
The time and distance interval between the emission of the current by each signal transmitter and the receipt of the altered current by the corresponding return signal sensor assembly together with the properties of the return signals such as their amplitudes and/or phases can be used to determine the conductivity/resistivity of the materials in the geologic units, that is, the ability of the materials to resist electrical currents. The resulting formation micro-resistivity can be recorded, for example, as a function of the tool's depth or position in the wellbore. This data is then later processed to create a micro-resistivity well image log showing different properties of the geologic units surrounding the wellbore.
For example, the recorded resistivity of the rock and other materials in the geologic units can be used to determine the nature of the rock and other materials. For example, the resistivity of shale is different than the resistivity of sand, and hydrocarbons and water will also impact the signal and resulting data. The resistivity data can be very valuable in the search for hydrocarbons and can dictate how the drilling and/or completion programs move forward.
A very important component of any image logging tool is the spatial control of where the transmitters and signals are oriented in xyz space relative to the wellbore and the Earth. As used herein and in the appended claims, the “Earth” means the planet Earth. Over the last several decades, tremendous advances have been made in this area with the use of gyroscopes mounted inside the logging tool. Gyroscopes allow the data to be corrected in xyz space relative to the wellbore and the Earth to greatly improve the data quality. The corrected data allows an image of the wellbore and the surrounding geologic units to be produced that can be “unwrapped” to create a two dimensional or three dimensional view of the inside of the wellbore. Such a well image log can provide information regarding, for example, the formation lithology, the nature of the bedding, the content of fluid in the formation, and the dip and azimuth of the surrounding rock. The ability to view processed data in two-dimensional or three-dimensional space reduces the impact of poor data collection or processing errors due to faulty receivers, hole washouts, excessive tool spinning, insufficient receivers, poor sampling or high logging speeds. Thus, the quality of the final well image log is significantly enhanced.
The ultimate goal of any image logging tool is to get an accurate representation of characteristics of the geologic units surrounding the wellbore. One measure of the quality of the representation that can be obtained is the signal-to-noise ratio (the “S/N ratio”) associated with use of the tool. Both the rock being penetrated and the logging tool used to record the data create noise, most of which is random and cannot be easily eliminated. Reducing the noise and maximizing the signal strength associated with any well logging tool is a primary objective in the design and use of the tool. Maximizing the S/N ratio of an image logging tool will also improve the final product.
The S/N ratio associated with an image logging tool can be increased, for example, by decreasing the logging speed, using an eccentric, offset or off-center arrangement of transmitter/sensor assembly pads, moving the transmitter/sensor assembly pads closer to the wellbore wall, increasing the number of signal transmitters and corresponding signal sensor assemblies attached to the logging tool, acquiring data in more accurate three-dimensional xyz space, and then later processing the data better in three-dimensional xyz space.
Due to the low S/N ratio associated with cased wellbores, micro-resistivity, acoustic and optical image logging tools are typically only effective in an open-hole (non-cased) wellbore. For example, when a metal casing has been cemented in the wellbore, the metal in the casing interferes with the electrical, acoustic or optical signals being sent and received by the tool. The highly conductive nature of the metal casing creates “noise” that can overwhelm both the tool and the rock signal to and from the tool. A solid casing of any type can make optical image logging tools worthless in looking at geologic or engineering features in the surrounding formation. For example, solid plastic and composite casings are opaque in nature which can negatively impact the performance of optical image logging tools. Optical image logging tools are also negatively impacted by opaque or otherwise dirty drilling fluids, even in open holes.
Drilling rigs are very expensive to own, rent and operate. When a well is being drilled or a drilling rig is otherwise in place, time is money. As a result, a great deal of effort is made to keep the drilling and completion process moving forward in a timely and cost-effective fashion. However, many problems can come up that slow the process and cost the operator time and money. For example, getting a logging tool stuck in an open wellbore before casing has been run can be very time consuming and otherwise counterproductive. For example, logging tools are often “fished out” of the wellbore by specialized subcontractors who are brought out to the well site on a rush basis. Fishing a stuck logging tool out of the wellbore can take several days of rig and subcontractor time to accomplish. A stuck logging tool of the type that contains an active radioactive source can activate regulatory requirements that the well be abandoned and filled with red cement (the red cement warns subsequent well drillers to stay away from the buried active radioactive source).
Depending upon the regulatory environment associated with the well, most completed oil and gas wells are ultimately cased (typically with a metal casing). As a result, electrical, acoustic and optic-based image logging tools are only useful before the casing is installed.
The nature of an open-hole environment can also negatively impact the performance of an image logging tool. For example, excessive mud-cake buildup on the borehole wall can interfere with the signals being transmitted and received by an image logging tool. For example, a permeable rock zone that absorbs drilling fluid may result in a thicker mud-cake buildup than an adjacent low permeability zone. Also, the nature of the drilling fluid in the wellbore of an uncased hole can interfere with the signals being transmitted and received by an image logging tool. For example, highly resistive or conductive drilling mud, including commonly used oil-based muds, can be problematic for micro-resistivity image logging tools. Logging in an oil-based mud hole with a micro-resistivity image logging tool can require more complex data collection and processing.
Also, due to the fact that micro-resistivity, acoustic and optical image well logging tools can generally only be used to evaluate the geology in unprotected open-hole environments, the tools are typically designed to be pulled out of the hole by a wireline cable at a relatively high logging speed, for example, at a logging speed of at least 1000 feet per hour (“FPH”), usually at about 1800 FPH, and sometimes up to 3600 FPH. When a well is being drilled, it is always important to get the well cased and otherwise completed as soon as possible. This is due primarily to the daily cost of having a drilling rig in place (even if the drilling phase is complete, the drilling rig is still often used to complete the well). Also, in many wells, it is important to case the wellbore or one or more portions thereof quickly due to changing well conditions. For example, in some cases, the wellbore wall is sloughing or the stability of the geologic units around the wellbore is otherwise decreasing with time. In order to prevent the wellbore from collapsing or caving in, a well operator may decide that casing needs to be put in place sooner as opposed to later.
Also, in an open-hole environment, the likelihood that changing pressures, changing borehole shapes and other conditions will cause an image logging tool to get stuck increases significantly at slower logging speeds. This problem is exacerbated by the outwardly biasing extendable arms and corresponding pads of modern micro-resistivity image logging tools which make it easier for such tools to get hung up on the rock wall, for example, due to deviations (“doglegs”) in the inclination of the borehole. As a result, wireline logging engineers operating in open-hole environments are typically encouraged to use logging speeds of at least 1000 FPH and preferably 1,800 FPH.
Unfortunately, for a given image logging tool in a given wellbore environment, the quality of the logging data decreases as the logging speed at which the tool is run increases. A faster logging speed means a lower S/N ratio and less collected data. Less collected data means a lower quality final image log. In order to accommodate faster logging speeds and maximize image quality, image logging tool designers and manufacturers have increased the sophistication of the tools, including the number of pads and sensor assemblies on the tools, which allows a higher sampling rate to be used. Although this addresses the problem with low S/N ratios, it also significantly increases the cost of the tools. For example, a sophisticated micro-resistivity image logging tool can cost over $500,000 today.
The high cost of sophisticated modern image logging tools also creates problems in and of itself. For example, due to their high cost, micro-resistivity image logging tools are not widely available and can be in limited local supply. As a result, such tools may not be available to wireline logging engineers for use in a timely manner on a well. For example, additional planning and transportation costs may be incurred if the only available micro-resistivity image tool is located in another state.
The increased sophistication and capability of modern micro-resistivity image logging tools is not always needed. For example, in some cases, the well operator only needs or desires geologic unit dip and azimuth data. If this is the case, modern micro-resistivity image logging tools are used in a “dumbed-down” mode. In other words, the same expensive micro-resistivity image logging tool is run in the same deteriorating downhole environment and records the same data, but only part of the data is processed and presented. This is very wasteful of the data acquisition time and costs, particularly in view of the risk of placing such an expensive tool into poor wellbore conditions and thereby risking the tool being stuck.